So the guy's problem is that he isn't good enough with his girl yet to maintain her attention and you guys want to blame that on the girl and say that because she is having a good time with another guy who is apparently a cool guy, he should dump her? I disagree.
Your issue is not the girl, it is you. It sucks, but you need to learn how to be more of a man. Luckily you have found this forum and that's what it is here for. Don't blame your issues on the other guys, the girls, their behavior or anything else, realise that you are the one in control of your actions and your outcomes. Take control of your relationship by taking control of your own actions and becoming the man that your woman craves.
Instead of rejecting the other guy and being angry at him you guys need to learn from what they are doing right and try to be more like them, while still being who you are. Don't copy them, but see what they do and do similar kinds of things in order to receive the same SPAM they do. If they are being flirted with a lot then they are doing something right and you should learn what that is.
Also, recognize that just as you flirt with people who you encounter in your day and even the other cute girls at parties while your gf is there, so will your girls flirt. We're human beings and it's what we do. It's not a crime, but treating someone badly because of it will make them resent you and ruin a lot of relationships before you realise what you're doing wrong unless you work at stopping it before it gets to that point.
As far as your LMR issue in your other thread, perhaps your issues are related. Perhaps you aren't treating your girl the way she needs to be treated and so she doesn't feel safe at being that vulnerable with you. These are both symptoms of less than ideal dynamic in your relationship and that is a result of your behaviors and beliefs. Time for some inner game work. Good luck!
